The only excuse I could think of is extended time out in space of a decade or so.

Right. The original intention of TNG's creators, which unfortunately got lost due to the revolving door of producers, was that the Enterprise-D was a deep-space research vessel designed to go as much as 15 years without returning to a home port. It wasn't supposed to be doing diplomatic milk runs to Federation members and neighbors like it ended up doing for most of the series, but instead was supposed to be far beyond the fringes of known space, years' travel from home -- somewhat similar to Voyager's situation, really, except on purpose.

So think about it. How many volunteers would you get for such a mission if everyone had to leave their families behind, or defer starting families, for 15 years or more? That's a huge chunk of a person's entire life. It's not something very many people would be willing to give up. And for those who did, going a decade and a half without family life could be extremely stressful and harmful to crew morale and cohesion. The only way such an extended deep-space mission could really be feasible is if the ship isn't just a ship, but a whole community, a small, self-sustaining city in space.

Also, keep in mind another thing that later producers forgot: this was meant to be a research vessel, not a military one, so its crew included a large complement of civilian scientists. It wasn't just Starfleet personnel and their families, not as originally intended. I like to think of it as a university village in space. It was supposed to be primarily a research vessel -- with enough Starfleet presence and weaponry to defend it if it became necessary, but never intended to go into combat except as an absolute last resort. Maybe you could find enough military personnel willing to commit to giving up 15 years of their lives, but you'd be harder-pressed to get civilian scientists to join such a mission.

And then there's the other abandoned element, the ability to separate the saucer and leave it behind with the civilians aboard while the Starfleet personnel went into battle in the engineering hull.

So the problem wasn't with the idea of families on the ship. That idea was very well thought out in terms of the creators' original intentions. The problem was with the way the later producers screwed things up by ignoring those original intentions and turning the E-D from a ship exploring strange new worlds to a ship that spent most of its time hanging around known space and going on diplomatic or political missions -- and bringing the saucer along into combat because the only miniature they had that could separate was too cumbersome to use regularly. And forgetting the civilian presence altogether except for Keiko.

I often think that the creators' original intention might've been better served if they'd had two (or more) ships all along -- a large civilian research vessel commanded by Captain Picard and its Starfleet escort (of one or more ships) commanded by Captain Riker. Not only would you have a clearer separation between the civilian and military functions, but you could've had more interesting tension between the scientists and defenders and their differing approaches to crisis situations. At the very least, Deanna should've been a civilian, to better represent that facet of the ship's intended complement.

I think this sums it up quite nicely.

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